For the projection of images, either parallel methods, as it is the case for example with LCDs (liquid crystal displays) or with micro-mirror arrays, or scanning methods are used, as they are for example realized by a biaxially movable or two uniaxially movable mirrors. Disadvantageously, in the parallel methods, a comparably large substrate area in manufacturing and also a complex test procedure are necessary. Both result in a comparably high price for parallel-type projectors, so that methods working in parallel are not considered for a low-cost projection apparatus.
In the scanning methods, the projectors include one or two movable mirrors enabling deflection of a light beam about two deflection axes and two-dimensional deflection of the light beam, respectively. By the deflection, the light point generated by the light beam is moved on the image field, the intensity of the light beam meanwhile being modulated on the image field and being dependent on the instantaneous projection place of the light point on the image field.
Thus, representation of the desired image content is such that a light source is modulated corresponding to the image point data of the image to be represented. The modulation here for example is via a change in the amplitude, wherein an amplitude value of the light beam is adapted corresponding to the brightness of the point to be represented. So as to achieve as many gray scales as possible, the laser should be capable of being modulated continuously or in a more or less stepless manner in its amplitude.
Projection systems can be realized inexpensively and with little spatial need if a microscanner-mirror-based laser system is used, for example. One example for such a laser system is described in U.S. Pat. No. 6,843,568. Here, light emitted from one or more laser sources is deflected by means of a micromirror swinging about two axes and projected onto a projection area or image field.
One main parameter of scanning projection systems is the scan method used. The most important scan methods are linear scan and resonant scan, with intermediate forms also being possible, but seldom used. Since the scan method can be chosen separately for both axes about which the emitted light is deflected, scanning projection systems can be divided into three groups:                1. Systems with linear scan for both projection coordinates        2. Systems with linear scan for one and resonant scan for the other projection coordinate        3. Systems with resonant scan for both projection coordinates.        
In systems with linear scan, the projection is based on a column- and line-like representation of the image. So as to allow for this column- and line-like representation, in these systems, the line frequency, i.e. the frequency of the deflection of the light beam and/or light point in horizontal direction, is large as opposed to the column frequency, i.e. the frequency of the deflection of the light beam along the vertical direction. This ratio of these frequencies with respect to each other determines the number of resolvable lines and can only be increased by a so-called interlace method, in which at first all even lines and then all odd lines of an image are scanned and/or represented alternatingly.
In micromechanically manufactured scanning projectors or scanners, the achieving of low eigenfrequencies or resonance frequencies poses a fundamental problem, since the mechanical stability of the system decreases with the eigenfrequency. If the vertical deflection is to be excited in resonance or resonantly, the deflection mirror or the deflection mirrors thus have to be operated at a correspondingly even greater horizontal frequency. Alternatively, the vertical deflection has to be performed in the quasi-static operation, in order to be able to resonantly create a horizontal deflection. In the case of a resonant vertical row deflection of the light beam, a problem is that the horizontal column frequency has to be great relative to an anyway great resonant row frequency. The great horizontal deflection frequencies occurring therein induce dynamic deformation of the mirror plate, which leads to resolution problems in the projection. In the case of the quasi-static vertical row deflection, very high operating powers are necessary that make miniaturization of the control of the deflection unit or the mirrors impossible or the deflection unit very expensive. These problems also cannot be eliminated by a decrease of both frequencies, since the row frequency or the vertical frequency determines the image repetition frequency, and a too low image repetition frequency leads to a flickering of the image.
FIG. 6 shows a schematic illustration for a linear scan. The projection area on which the image is represented is in the xy plane, for example, and is limited by a right edge 40a, a left edge 40b, a lower edge 40c and an upper edge 40d. A light beam and/or a laser 41, for example, starts at a starting point A in the upper right image corner, and the projection of the image takes places by movement of the light beam 41 by means of the pattern represented. The light beam thus moves at first from the right image edge 40a to the left image edge 40b, with only movement along the x coordinates taking place, then the light beam 41 moves to a next line, i.e. it moves back to the right image edge 40a, but with the y value being changed such that the light beam 41 appears in a following line of the image to be represented. From the right image edge 40a, the light beam again moves in parallel to the lower image edge 40c towards the left image edge 40b. The pattern continues correspondingly until the light beam 41 has represented the entire image line by line, and an end point A has been reached.
In a linear scan, the deflection of the light beam or the laser 41 in the respective projection direction is in time-linear manner. With this, all image points with equal time period are projected. This allows for especially simple readout of the image projection data and uncomplicated modulation independent of the position of the respective image point. At the same time, however, high demands are placed on the deflection system with respect to its linearity. This means that the light beam 41 moves exactly along the row line (i.e. in parallel to the x direction) and moves along the lines at a speed as constant as possible, so that the light beam reaches the left image edge 40b at fixedly prescribed time instants. The control of the deflection unit necessitates, as shown in FIG. 6, a sawtooth or triangular signal, i.e. a signal containing many harmonics. As already explained in detail, the frequency with which the mirror deflects the light beam along the x direction strongly differs from the frequency with which the mirror deflects the light beam 41 along the y direction, and are given by the number of represented lines of the image or the repetition frequency of the image in one second. Usually, these frequencies are default externally and are not dependent on corresponding resonance frequencies for the deflections of the mirror in the two directions of the xy plane. The control of the mechanical system thus is against the resonance behavior, which means a relatively high energetic effort.
In the resonance scan, the deflection of the laser 41 takes places according to a sine function, wherein the frequency may for example be adapted to the resonance behavior of the mirror. Since the laser 41 in this case does not move across the projection area at constant speed, the projection durations of the image points hence are position dependent. In systems with resonant scan for both projection coordinates, the coverage of all image points may for example also be achieved by the realization of a Lissajous figure with high repetition time. Great repetition times here mean a great least common multiple for the coordinate projection frequencies. In this method, however, the readout of the image projection data is made complicated.
The non-uniform pixel modulation times, which for example are a result of differently long projection durations of the image points depending on the position, necessitate adaptation of the (laser) modulation explained in greater detail in the following. As already described, in a resonance scan, it is advantageous to perform the control of the system by means of pure sine signals. Except for the fact that, spectrally speaking, pure sine signals are ideal, the mechanical resonance properties of the deflection systems may thereby be utilized to achieve energetic optimization.
The modulation in a double-resonant scan may for example take place as follows. In the projection of an image constructed of a rectangular pixel raster by means of a laser, the laser source is operated at constant power during the entire sweep of the area of a pixel (=time duration Tpixel), so that the energy Epixel (amount of light) of the image point to be represented is radiated integrally over this time period. Since the brightness of the image point may vary continuously, it also is desirable to modulate the laser power in analog or very fine manner.
FIG. 5 shows an example for a conventional amplitude modulation. An image having 16 pixels is illustrated on the left-hand side. Except for a pixel A and a pixel B, all further pixels are white. The pixel A has a lighter gray scale, and the pixel B a darker gray scale. The laser beam 41 reaches the pixel A at a first time instant T1 and sweeps the pixel A until a second time instant T2. The pixel B is swept in a time period between the second time instant T2 and a third time instant T3. Depending on the speed of the laser beam 41 and depending on the pixel size, the sweep time duration or the dwelling duration Tpixel of the laser beam 41 for the respective pixel may vary.
The right-hand side of FIG. 5 shows how the power of the laser ilaser is varied correspondingly, in order to represent the image of the left-hand side in FIG. 5. Until the first time instant T1, the laser radiates at a constant maximum power imax, which corresponds to the white pixels. At the first time instant T1, at which the laser beam 41 reaches the pixel A, the power of the laser is reduced from the value imax to a value i1. Between the first time instant T1 and the second time instant T2, while the laser sweeps the pixel A, the laser constantly radiates at the first intensity i1. At the second time instant T2, the laser beam reaches the pixel B, and the power of the laser beam is reduced from i1 to a further value i2. The second intensity i2 of the laser again remains constant until a third time instant T3, at which the laser beam leaves the pixel B.
Since it is assumed here that the time instants T1, T2 and T3 are at the same or almost the same distance to each other and since the second intensity i2 is lower than the first intensity i1, the corresponding pixel B is darker than the pixel A. From the third time instant T3 onward, the laser again radiates at the maximum power imax, so that the following pixel again corresponds to a white image point. The brightness with which an image point or a pixel appears on an image field corresponds to the amount of light or energy the laser transfers during the sweep time duration Tpixel and can be calculated as follows:Laser modulation: Tpixel·ilaser=Epixel,
wherein ilaser is constant in the interval [T1, T2] and Tpixel=T2−T1=T3−T2 is assumed.
Hence, in conventional laser projection systems, the brightness modulation only is via gray scales of the laser. So as to be able to project a realistic image, the amplitude of the laser (and hence the radiation intensity) should be regulatable in analog or very fine manner. However, this often cannot be realized technologically, or only with great effort. In particular, a technical problem arises because the sweep time duration Tpixel may lie in the range of only few nanoseconds, depending on resolution. This necessitates the use of lasers capable of being amplitude-modulated in quasi-stepless manner, having a modulation frequency of several 100 MHz.